First rapid planet-reader makes unique family portrait

The four spots lettered b to e are planets. The star is at the centre of the blacked-out circle. This is a composite image using 30 wavelengths of light and was obtained over a period of 1.25 hours in June 2012

(Image: Project 1640)

An exoplanet portrait snapped from Earth has revealed the chemical contents of four worlds at once. It is a feat that could mark the beginning of rapid characterisation of planets around other stars.

Spectral measurements split light coming directly from planets into different wavelengths to reveal the signatures of molecules in their atmospheres. This makes spectra important clues to exoplanet features, including habitability.

“The study of exoplanets really has been starved for spectroscopic data, which is the most revealing data you can get on a new type of object,” says Ben Oppenheimer of the American Museum of Natural History in New York City.

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The new spectral picture comes from Project 1640, an international collaboration named in honour of the wavelength of infrared light at which it works best – 1640 nanometres. Using the Hale telescope at the Palomar Observatory in California, project members collected data on the star HR 8799 and its planetary system of four massive worlds.

Meet the family

The planets are young, probably between 30 and 100 million years old. They still glow with the heat of their formation at about 1200 kelvin, and they are all giants, between five and 13 times Jupiter’s mass. Despite these similarities, their spectra revealed a family of very different worlds. Some contained signs of water and methane in their atmospheres, while others gave whiffs of hydrogen cyanide or acetylene.

Previously, only a handful of planets have had their spectra measured, and most of those were transiting planets&colon; worlds crossing in front of their stars as viewed from Earth. The outermost planet of HR 8799 has been measured before using this method.

Project 1640 instead uses advanced imaging techniques that block the light from the star, leaving only planetary signals, including worlds that do not transit. This means they can detect signals from all of the planets in a system simultaneously.

Origin stories

Adam Burrows of Princeton University, who was not involved in the new work, says the team has not yet ruled out all the possible observational errors that can crop up in spectral studies. More measurements and higher-resolution data could iron out the uncertainties, he says.

“But barring that, it seems there is some character among planets, that they are similar in some respects but different in crucial ways, that suggest something about their origin,” Burrows says, although he is not yet sure what that might be.

Many more spectra will be on the way. Last summer, Project 1640 began a three-year campaign to find and characterise more planetary systems like HR 8799 by surveying 200 stars within 150 light years of our solar system. And two similar projects, the Gemini Planet Imager and Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE), are expected to start in the next year or two.

“Spectra are the means by which you really characterise these things,” says Burrows. “Project 1640 is giving us a foretaste of what is possible now and what will be possible in the future.”